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Threads about the issue began turning up on Apple's Discussions support boards back in November, shortly after the new unibody MacBook and MacBook Pro began shipping. Customers of both models (1, 2) reported that when wearing non-Apple headphones, even the slightest bump or movement of the cord — such as turning one's head to the side — would result in a disconnection that caused audio to be rerouted back through the notebook's built-in speakers.
For those connecting external speakers to their MacBook, the mere act of setting a book down on the desk next to the computer would reportedly disrupt the connection. Upon closer inspection, it turns out that some 1/8th-inch miniplugs are left with a tad more wiggle room while in receptacle, namely those lacking a fourth conductor.
"When a jackplug is inserted into the socket there is a degree of play in the socket," wrote forum user movetolearn. "Fully pushed in...it works. Give a gentle pull, the plug moves back out maybe 0.5mm and, even though the plug stays in the socket, the sound comes out of the speakers."
As time went on and posters tested additional playback devices, they discovered longer plugs with four contacts and three separator rings would sit firmly and securely, while traditional stereo jacks with three contacts and only two separator rings resulted in a loose connection.
"One thing I notice is that my iPhone 3G headset works fine," wrote user Toronto-Ty. "What I am thinking is that since the plug has integrated stereo l/r and mic, there are 3 plastic bands around the metal plug, thus creating 4 metal sections. The fact that the headphone jack on the new MacBook is integrated mic as well, is what I feel the problem might be."
The same poster later reported a secure connection using Apple's in-ear headphones with mic. He also claims to have e-mailed Steve Jobs about the matter, only to receive a response from the company co-founder essentially telling him he was "wrong, literally."
Last year the company transitioned a number of its headphone jacks to quad-conductor jacks with microphone capabilities. Users who are experiencing the loose connection issue with third-party headphones on their new MacBooks may want to explore iFrogz $4 adapter solution suggested by one forum member.
For more on the differences between Apple's various audio and video cable connectors, see the article titled Using iPod & iPhone Video Out: Background and In-Depth Review.